In order to implement a secure and safe society, it is a fundamental and indispensable element that various types of products, such as aircraft, truck, pipes of electric power plant, bridge, and an implant used in orthopedics and dentistry, have mechanical safety. Thus, stress and strain measurement technology plays an important role in implementing and evaluating product mechanical safety.
In stress distribution measurement, there has hitherto been used a strain gauge technique by which many strain gauges are attached to the surface of an object to be measured and stress/strain distribution is perceived from output signals of each strain gauge when a load is applied to the object. Using this technique, quantitative measurement can be performed, but the number of gauges attached is limited in a wide ranging measurement area or in a narrow measurement area; thus measurement having no missing point cannot be performed.
Accordingly, there has been developed and put into practical use a thermoelastic stress measurement technique (also called an infrared stress measurement technique) by which a minor temperature variation caused by thermoelastic effect when a load is applied to an object is measured by use of infrared thermography to visualize surface stress distribution. This technique is characterized by allowing two-dimensional measurement of surface stress distribution (the change in the sum of principal stresses) in a manner having no missing point, independently of whether it is a wide ranging area or a narrow area.
Meanwhile, the present applicant and others have succeeded in manufacturing, based on the research on inorganic materials emitting light according to mechanical energy, a material composed of a base material being a piezoelectric material of particularly a wurtzite structure and an inorganic material of luminescence center, as shown in Patent Document 1 described below. The patent application has been filed as a result of finding out that when this is added to the above base material, the light emitting intensity of the resultant thin film can be dramatically improved. Thereafter, as a result of further research, various inorganic materials emitting light according to such force have been found out, and at the same time the research on the use of this material in various fields has also progressed. For example, as disclosed in Patent Document 2 described below, it has been proposed that a mechanoluminescence material is preliminarily mixed into concrete in order to detect an abnormal stress produced before the concrete is broken.
Patent Document 1: JP Patent Publication (Kokai) No. 11-120801 (1999) (JP Patent No. 3265356)
Patent Document 2: JP Patent Publication (Kokai) No. 2003-137622
As described above, the thermoelastic stress measurement technique is characterized by allowing a surface stress distribution of an object to be two-dimensionally measured in a manner having no missing point. This is because, as indicated by formula (1), if a small temperature change ΔT occurring when a load is adiabatically applied to an isotropic elastic object is measured by thermography, the change in the sum of the principal stresses Δσ having a proportional relation with the temperature change ΔT can be measured as an image. However, from its principle, there is an essential limitation that only the change in the sum of the surface principal stresses (Δ(σ1+σ2)) can be measured as physical quantity, and furthermore respective principal stress components are unknown, and pure shearing stress acting on the object cannot be measured because it causes no temperature change.ΔT=−k·T·Δσ  (1)
ΔT: the change in temperature (K)
k: the thermoelastic constant (1/Pa)
T: the object temperature (K)
Δσ: the change in the sum of the principal stresses (Pa)
Here, thermoelastic constant k is given by formula (2).k=a/(ρ×Cp)  (2)
a: the coefficient of linear thermal expansion (1/K)
ρ: density (kg/m3)
Cp: the specific heat at constant pressure (J/(kg/K))
Thus, an object of the present invention is to provide a stress analysis method and stress analysis apparatus by which not only thermoelastic stress measurement but also stress measurement using a mechanoluminescence material are used in combination, whereby stress measurement can be performed more in detail while exceeding the principle limitation of thermoelastic stress measurement technique.